A passive-optical network (“PON”) is a fiber optic network to which users are connected through passive rather than active devices. PONs are commonly used to provide high speed information services such as voice, data and video in local access networks, local area networks and wide area networks. A PON system includes an optical line terminal (“OLT”), typically located in a central office of a telecommunications provider, multiple optical network units (“ONUs”), typically located near end-user subscribers, and an optical fiber transmission line to which the OLT and the ONUs are connected for delivering information services to the subscribers. These optical networks do not require an optical-to-electrical-to-optical conversion at each node, and instead rely on passive optical components which are considered more reliable. PONs are also less expensive than other types of networks and allow use of multiple optical wavelengths to increase network bandwidth.
Conventional PON systems employ point-to-multipoint tree, star, or bus architectures. However, tree and bus architectures do not protect against certain network failures such as those caused from fiber cable cuts or component failures, whereas star architectures may demand relatively greater amounts of fiber cable. In a typical tree PON 10, as shown in FIG. 1, an OLT 12 connects via an optical transmission line 14 to a 1:N passive optical splitter 16. In turn, the optical splitter 16 splits an optical signal provided by the OLT to several ONUs 20-26. With sufficient optical power supplied to its input, the 1:N splitter can be of any size N. The available optical power budget is determined by the strength of the laser diode transmitters, the insertion loss of the passive network including fiber, connectors and couplers, and the receiver sensitivity. The amount of optical fiber deployed in the outside plant can be reduced by locating the 1:N splitter centrally among the ONUs, although it is often convenient to place the 1:N splitter within the Central Office at the OLT location. Point-to-multipoint PONs, such as the tree configuration described in FIG. 1, allow a single laser within the broadcast Optical Line Terminal (OLT) to be shared among N Optical Network Units (ONUs).
If the ONUs are more widely distributed, then the conventional PON bus network 30 as shown in FIG. 2 is often used. An Optical Line Terminal 32 is connected to an optical transmission line 34. On the transmission line are a number of sequential optical tap couplers referred to simply as couplers (“C”). A fiber optic coupler (or splitter) is a passive optical component with three or more fiber ports that split the optical signal among fiber ports in a ratio that can be specified. The lightwaves are split in a similar manner to a beam splitter in bulk optics, but done within the optical fiber itself. Optical couplers are typically made from a fused fiber biconical taper method or by aligning and fixing fibers to waveguide couplers. A coupler 36 splits off a portion of the signal power coming from the OLT 32 and transmits it to ONU1 38. The remaining portion of the optical signal from the OLT 32 continues on down the bus to coupler 40 which splits more signal power off to ONU2 42. The bus may be configured to any length, provided that sufficient signal power is available. Optical signal repeaters may be used on the bus to allow for extension of the bus. Any of the ONUs can transmit back to the OLT 32, and their signals are coupled to only travel in the direction toward the OLT so as not to interfere with the other ONUs on the line.
The tree and bus PON networks are not protected against cable cuts and they require one or more fibers per ONU.
Several types of tree and bus PON networks have been proposed using a signal containing single and/or multiple discrete wavelengths by the use of wavelength division multiplexing. Shown in FIG. 3 is an example of one such network 60. Within the OLT 62, a series of optical transceivers 64-70 (transmitter and receiver) represent a series of optical channels that are multiplexed via a wavelength division multiplexer (“WDM”) 72. The multiplexed signal is routed through an optical trunk transmission line 78 to a passive splitter 80. The multiplexed signal is split off as a tree network to a series of ONUs 84-90 that each contain a filter or internal WDM for extracting desired wavelengths. The ONUs may send a multiplexed signal which passes back to the demultiplexer of WDM 72. Using wavelength division multiplexing, a number of optical channels can be supported by a transmission line having a single optical fiber.
Protection against cable cuts can be found with conventional “active” optical networks employing a ring architecture, such as Synchronous Digital Hierarchy/Synchronous Optical Network (SDH/SONET) and Fiber Data Distributed Interface (FDDI) networks. These ring based networks are “active” in the sense that, at each network node, an optical-to-electrical-to-optical (O/E/O) data regeneration is performed. Each active network node within a SONET network is referred to as an “add/drop multiplexer” (“ADM”). An exemplary SONET ring 100 is shown in FIG. 4. The ADM 102 contains twin pairs of transmitters and receivers which connect on one side of the ring to the pair of optical transmission lines 104, 106 that are connected to a transmitter and receiver pair for another ADM 108. Each ADM 108, 114, 120, 126, 132, 138, 144 within the ring acts as a repeater by re-transmitting the signal it receives in the same direction as received, thereby transmitting the signal around the ring. Optical transmission line segment pairs extend between each pair of adjacent ADMs and comprise the optical ring. When the cable is cut at any one place within the ring, these active networks can route the signal on an alternate path within the ring to effect what is termed “self-healing.” However, each ADM within the ring performs data reception and data regeneration, therefore a failure within an ADM can disrupt overall communication in the ring. These active ring networks are symmetrical networks operating at a single line interface rate. SDH/SONET networks are highly-survivable optical based networks most often configured with a dual ring architecture of the type shown in FIG. 4. This architecture is more flexible than a point-to-point configuration, and allows for a full redundancy and virtually instantaneous fault protection. With built in node regeneration, SDH/SONET rings are primarily used in long-distance and metropolitan areas.
Conventional passive optical networks do not provide the redundancy that is necessary in a number of environments while the SONET active-ring networks, although “self-healing,” are costly due to the signal regeneration requirements and are not protected against failures within the regenerative ADMs used within the optical ring.
Accordingly, and as recognized by the inventors hereof, a need exists for a network having the fault tolerance of an active ring network, and the reliability and lower cost of a passive optical network.